It has previously been proposed in the prior art to make use of angular modulation, for example frequency modulation, in the transmission of digital information to a means for processing that information. When the processing means comprises an on-board module which is arranged to receive signals from a radio frequency telecontrol device, i.e. a remote radio transmitting device, for commanding the opening or closing of the doors of a vehicle, the relatively high current consumption of the on-board module presents major problems. In this connection, the on-board module has to remain on alert all the time, so that it can shift into its active mode at any time, on reception of a suitable radio frequency signal.
The device located at the input of the on-board module has to be capable of amplifying the signals received on a receiving antenna, the gain of which is necessarily limited because of the small amount of space which is available for it.
In addition, the transmitting device is located in a portable key carried by the user of the vehicle, and this key includes an emitter the output signal power of which is itself very weak, because the transmitting device also has to include an energy source, an electronic generating circuit, and a transmission antenna, all in a very small format. This format may be for example similar to that of a credit card, or it may be contained in the head of a mechanical key associated with the anti-theft system of a motor vehicle.
The input stage of the on-board module must therefore consist of a very high performance electronic circuit, the current consumption of which must, on the other hand, be as small as possible.
Circuits having low current consumption do of course exist per se. However, their selectivity is correspondingly reduced, which makes them unsuitable for overcoming the above mentioned problem.
In addition, it is known to provide an input device of the intermediate frequency kind, but this has to have at least two stages in order to effect both the selection and the decoding of the modulation received from the transmitting device. As a result, the current consumption of such an input device is high, because it is necessary to polarise two successive stages.
In a co-pending patent application, the present Applicants have proposed an arrangement, having a plurality of stages, for an input device in which electrical current consumption is considerably reduced.